1. Field of the Invention
The present invention relates to an apparatus and a method for manufacturing a semiconductor device, and particularly to an apparatus and a method for accurately forming a silicon oxide film on the surface of a semiconductor substrate.
2. Description of the Background Art
A silicon oxide film formed by oxidizing a polysilicon film (hereinafter, referred to as "polyoxide film") has been used, for example, as an insulating film between a floating electrode and a control electrode of a flash memory. A known method of forming a polyoxide film includes the steps of disposing a polysilicon film in a vertical batch type oxidizing furnace and supplying an oxidizing gas to the surroundings of the polysilicon film, to oxide the surface of the polysilicon film.
FIG. 3 is a configuration diagram of a related art vertical batch type oxidizing furnace for forming a polyoxide film. The related art vertical batch type oxidizing furnace includes gas flow rate control units 10, 12 and 14 for controlling flow rates of nitrogen gas, oxygen gas and hydrogen gas, respectively. The gas flow rate control units 10, 12 and 14 are communicated to an external burning pipe 16. A burning heater 18 for burning hydrogen gas and oxygen gas to generate steam is provided around the burning pipe 16.
A reaction chamber 22 is connected to the external burning pipe 16 via a gas inlet pipe 20. A heater 24 for heating the gas inlet pipe 20 and the reaction chamber 22 is provided around the gas inlet pipe 20 and the reaction chamber 22. An exhaust passage 26 is provided in the reaction chamber 22. A shutter plate 28 for sealing the inner space of the reaction chamber 22 is provided at the bottom portion of the reaction chamber 22.
The related art vertical batch type oxidizing furnace further includes a wafer boat 32 for holding a plurality of semiconductor wafers 30. The wafer boat 32 is fixed on a boat lifter 36 by means of a boat holder 34. The boat lifter 36 is adapted to carry the wafer boat 32 in the reaction chamber 22 in a condition where the shutter plate 28 is opened, and to seal the inner space of the reaction chamber 22 from the external atmosphere, that is, the atmosphere of a clean room.
A related art method for forming a polyoxide film will be described below with reference to FIG. 4. FIG. 4 is a flow chart showing sequential processing steps carried out for forming a polyoxide film using the related art vertical batch type oxidizing furnace.
In accordance with the related art method, at Step S100, the semiconductor wafers 30 held on the wafer boat 32 are inserted in the reaction chamber 22 by the boat lifter 36. At this time, the reaction chamber 22 has been already heated at about 700.degree. C. by the heater 24. In the insertion stage of the semiconductor wafers 30, a mixed gas of oxygen and nitrogen or an oxygen gas is supplied in the reaction chamber 22 while the flow rate of the gas is controlled at a specific value by the gas flow rate control units 10 and 12 or the single gas flow rate control 12.
By supplying the oxygen containing gas in the reaction chamber 22 in the insertion stage of the semiconductor wafers 30 as described above, organic matters adhering on the surfaces of the wafers can be removed by oxidation. Accordingly, with the above-described insertion treatment, it is possible to enhance uniformity of the thickness of the polyoxide film over the entire surface of each wafer.
After insertion of the semiconductor wafers 30 in the reaction chamber 22, at Step S102, the state in which the above gas is supplied in the reaction chamber 22 is kept for a specific period of time for stabilizing the wafer temperature.
The process goes on to Step S104, at which the reaction chamber 22 is heated to an oxidizing temperature, specifically, about 900.degree. C. by the heater 24.
After the temperature of the reaction chamber 22 reaches the oxidizing temperature, at Step S106, an oxidizing gas is supplied in the reaction chamber 22 via the gas inlet pipe 20, to oxidize the semiconductor wafers 30. At this moment, the flow rate of the oxidizing gas is controlled by the gas flow rate control units 10, 12 and 14, and also the oxidizing gas is heated by the external burning pipe 16. The oxidizing gas may include steam gas produced by reaction of oxygen with hydrogen in the external burning pipe 16, or oxygen gas.
After completion of the oxidizing step, at Step S108, the gas to be supplied in the reaction chamber 22 is changed from the oxidizing gas to nitrogen gas, so that the atmosphere in the reaction chamber 22 is substituted for the nitrogen gas.
At Step S110, the semiconductor wafers 30 are held in the reaction chamber 22 until the temperature of the semiconductor wafers 30 is lowered to a specific value.
After the temperature of the semiconductor wafers 30 is sufficiently lowered, at Step S112, the semiconductor wafers 30 on the wafer boat 32 are taken out from the reaction chamber 22 by the boat lifter 36.
In accordance with the above-described related art method, since the surfaces of the semiconductor wafers 30 are exposed to atmospheric air in the clean room during the period in which the semiconductor wafers 30 are placed outside the reaction chamber 22, organic matters and the like in atmospheric air may adhere on the surfaces of the semiconductor wafers 30. As described above, the organic matters can be removed to some extent by supplying oxygen into the reaction chamber 22 in the insertion stage of the semiconductor wafers 30.
In accordance with the related art method using the related art vertical batch type oxidizing furnace, however, the semiconductor wafers 30 are inserted in the reaction chamber 22 in the state in which the interior of the reaction chamber 22 is opened to atmospheric air in the clean room. In the insertion stage of the semiconductor wafers 30, the concentration of oxygen gas to be supplied in the reaction chamber 22 is generally different from the concentration of oxygen in atmospheric air. As a result, according to the related art method, it is difficult to accurately control the oxygen concentration in the reaction chamber 22, more specifically, to equalize the oxygen concentration in the reaction chamber over the entire region in the insertion stage of the semiconductor wafers 30.
When the semiconductor wafers 30 held on the wafer boat 32 are inserted in the reaction chamber 22, those held near the top portion of the wafer boat 32 pass through the interior of the reaction chamber 22 for a longer distance as compared with those held near the bottom portion of the wafer boat 32. Accordingly, if there occurs a variation in oxygen concentration in the reaction chamber 22 in the insertion stage of the semiconductor wafers 30, the oxidizing rate and the ability of removing organic matters adhering on the semiconductor wafers 30 in the insertion stage of the semiconductor wafers 30 are dependent on the positions of the semiconductor wafers 30. Such dependence on the positions of the semiconductor wafers 30 causes deterioration of uniformity of the polyoxide film. In this way, the related art method has a problem that it is difficult to stably manufacture polyoxide films having uniform qualities over the entire surfaces of the semiconductor wafers 30.
Also in accordance with the related art method, the semiconductor wafers 30 are heated to the oxidizing temperature in the oxygen containing atmosphere. To be more specific, since the temperature rise of the semiconductor wafers 30 is performed in a high temperature region of 700.degree. C. or more in the oxygen containing atomosphere, a slight oxide film is formed on the surface of each semiconductor wafer 30 before the oxidizing treatment performed at the oxidizing temperature of about 900.degree. C As a result, in accordance with the related art method, it is difficult to form an extremely thin polyoxide film on the surface of each semiconductor wafer 30.